The goal of this project is to determine the control and organization of human spinal circuits that help coordinate movement and to understand possible malfunctioning of these circuits in movement disorders, particularly those with excessive muscle contraction: Stiffperson syndrome, dystonia, and spasticity. Progress was made in three areas during FY2000. First, we found that the motor cortex is hyperexcitable in patients with Stiffperson syndrome, using the technique of paired pulse transcranial magnetic stimulation. This may reflect loss of inhibitory cortical interneurons in this disorder. We found that vibration, a stimulus that induces low frequency depression of the Ia stretch receptor afferents, produces increased cortical potentials in normal subjects only when it is sufficient to produce an increase in spinal excitability. This lays the groundwork to study why patients with focal dystonia are particularly sensitivity to vibration: is it caused by alterations in processing sensory inputs at the cortical or spinal level? Lastly, we have begun assessing whether patients with spasticity exhibit changes in low frequency depression of stretch reflexes, with a special emphasis on disorders with corticospinal degeneration.